Micropipette pullers are known devices which are used to draw a capillary tube of a first diameter to have a portion with a much smaller diameter. An example of such a device is shown in U.S. Pat. No. 4,600,424. In such a device, a capillary tube of a type that is widely available commercially, is placed in the micropipette puller where it is held between two grippers. A central portion of the capillary, located between the grippers, is heated to a temperature at which the capillary is sufficiently soft to be pulled without breaking, but kept below a temperature at which it will sag under its own weight. Typically, heating of the capillary is accomplished using a filament comprising a metal band or wire that encircles the capillary. The grippers are then pulled apart causing the central portion of the capillary to stretch, thereby decreasing its diameter. The resulting reduced-diameter central portion of the capillary remains hollow. It is possible to obtain a micropipette having an internal diameter of 0.1 micron or less in this manner. When sufficiently pulled apart, the central portion of the micropipette separates, resulting in the formation of two micropipettes with extremely small hollow tips. These are especially useful in biological research where the micropipettes serve as microelectrodes that can be inserted directly into cells or other very fine biological structures and used, for example, to measure intracellular electrical phenomena. Alternately, the puller may be used in a manner which does not cause in separation of the micropipette, resulting in a tube having an "hourglass" configuration. Such a tube may then be cut in two. Such a technique is required when making micropipettes with a larger internal diameter than would result when the capillary is stretched to the point of separation.
The shape of the tip of the microelectrode is a significant factor in certain research applications and it is thus important to be able to reproducibly manufacture micropipettes with a desired profile. For a given capillary, the shape is a function of shape of the filament used in the puller, the proximity of the filament to the capillary, the temperature of the capillary when it is pulled, and the speed with which it is pulled. For example, a wide filament is used to create a long micropipette. Likewise, a filament that is held in close proximity to the capillary will result in a sharp heat gradient between the heated and unheated portion of the capillary. This will cause an initial fast taper of the capillary as it is pulled. If the filament is further from the capillary, the heat gradient will be more gradual and the micropipette will have a more uniform taper from its shoulder to the middle of the central portion.
The better micropipette pullers are now all computer controlled so that the variables in the pulling process, such as the filament temperature, length of the pull, the strength of the pull and the rate that the capillary is cooled, may be programmed into the puller. The only important variable which is not easily changed is the type of heating filament which is used. Efforts have been made to move the filament during the pulling cycle in order to heat a greater area, but the movement if the filament was slow and very difficult to control in a repeatable manner.
In many applications, the micropipettes and microelectrodes made using a puller of the type described would be enhanced if made of quartz. For a great many applications, quartz has better mechanical, optical, chemical and electrical properties than glass. For example, quartz is stronger, has a lower dielectric constant, is freer of contaminants, and is less fluorescent than glasses typically used for making capillaries. Unfortunately, quartz has a relatively high melting point (2100.degree. C.), which is too high to permit softening by filaments. For example, a typical material used as a filament, platinum-iridium, has a melting point of 1815.degree. C. Moreover, filaments evaporate during use which, even if the evaporation rate is low, may cause undesired contamination of the micropipette or microelectrode formed therefrom. The rate of evaporation increases as the temperature of the filament approaches its melting point.
Recently, there have been disclosed different embodiments of micropipette pullers utilizing a laser as a source of heat. A laser can generate a much hotter temperature since it is not limited by its own melting point. Thus, a laser is ideal for heating quartz to a working temperature. Moreover, the contamination problem described above, due to filament evaporation, is completely avoided. When using a laser, however, it becomes necessary to include means for distributing the laser beam, i.e., the source of heat energy, along a desired length of the capillary.
Two mechanisms for distributing the heat from a laser beam have been described in the prior art. In one mechanism, the heat is distributed using a stepper-motor controlled mirror which oscillates the laser beam along the length of the capillary. In the second, a lens system, such as a zoom lens or a shuttle assembly of different fixed lenses, is used to distribute the laser beam in a desired pattern.
These laser beam distribution mechanisms, while providing superior performance, are costly to build and must be maintained in good alignment. It has been found that there is a market demand for a lower cost yet reliable solution to the problem of controlling the heat distribution pattern of a laser beam used to heat a capillary in a micropipette puller.
Accordingly, it is an object of the present invention to provide a micropipette puller using a laser as a source of heat, which is able to control the area of the capillary which is heated.
Another object of the present invention is to provide a low cost means, in a micropipette puller, for controlling the portion of the capillary that is heated by a laser.
A further object of the present invention is to provide a micropipette puller which is able to reproducibly manufacture quartz micropipettes of a desired configuration.
Other objects and advantages of this invention will become apparent to those skilled in the art from the following description, particularly when read in conjunction with the accompanying drawings.